I've been reading the Quick Primer for Old School Gaming and I
have found it enlightening, to say the least. One of the issues it brings up is the use of skill checks to determine whether or not somebody does something "successfully." Instead, it suggests that the players describe exactly what they do in order to achieve a desired result, such as searching for a trap.

How should I deal with "skill" based challenges in a viable way? For example, somebody wants to leap onto the ground and somersault to avoid fall damage. Or another player wishes to slide down a bannister into a group of unsuspecting monsters, sword at ready.

These sorts of things would require some skill, and could possibly end badly. The guide mentioned a "to-hit" roll being of possible use, and also percentages. Could these methods be briefly explained? Are there other methods available?

I don't have a decent reference available, but some GMs would just call the player to roll 1d20 and try and get lower than whatever ability score seemed most relevant to the task being attempted. Of course, this only makes sense if the task is one for which the outcome isn't obvious, as the 3-18 stat range means that failure and success are both always possible on such a roll. (Come to think of it, other dice sizes and die combinations might avoid that problem.)
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GMJoeMar 27 '14 at 1:22

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To add more of a risk/reward flavor to skill checks, I wonder if players could be allowed to adjust (within reason) the difficulty rating they have to beat with their skill check roll. Successfully beating a higher DR = a proportionately more successful outcome (eg, the PC successfully avoids all damage when they leap and somersault, or slides down the bannister and knocks down the group monsters). Or the DM could simply reward the player with bonus XPs for taking on more risk and beating the odds.
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RobertFMar 27 '14 at 16:05

3 Answers
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I touched on this in your broader question, in that basically much of it comes down to two options:

If the action isn't something critical to the storyline or something entertainingly done (e.g. your example of trying to avoid fall damage) you would simply decide for yourself what the chances are of accomplishing the task, and roll (or have the player roll) percentile dice. (Though in the case of your example, I'd also assign a definite chance of failure which causes extra problems, so that players know there's a risk to going outside the rules.)

If it's something critical (i.e. "if we don't convince these people to help, our plans grind to a halt") or something entertaining (like your bannister example), you can simply rule success (in the latter example, perhaps a higher chance of achieving surprise, or if you're feeling generous an automatic one).

This really comes down to style and whatever your particular value of "common sense" happens to be; it's also possible to entertain suggestions from the players as to what they think their chances might be, and make up your mind from there. With 1e, there are a lot of seat-of-the-pants judgement calls like that, and the idea is to both keep the game flowing and to encourage creative thinking on the part of the players without simply giving them everything. This is one thing very different from later iterations of D&D, which started to incorporate a greater number of rules to cover such things.

If you want some kind of rules, you can always base a number of these things on a relevant stat check...perhaps your person trying to tumble will need to roll vs. Dexterity. The roll itself can be varied according to difficulty...for instance, if you judge the task to be quite hard, perhaps they have to roll Dex or less on percentiles; for something hard but not extreme 2xDex, for something of middling difficulty 3xDex, and so on. (5xDex is equivalent to rolling on d20, BTW.) You can assign chances of disastrous failure as being anywhere from 10-33% of the failure range, according to your judgement of how likely a failed check is to end especially badly. For instance, if our tumbler has to roll 2xDex to succeed, and they have a 15 Dex, you might decide that their disaster range is equal to 20% of the failure range. The resulting d% roll could be something like 01-30 Success (less or no damage), 31-86 Normal failure (take normal falling damage), 87-00 Disaster (take 150% falling damage). It's also possible to assign "critical success" ranges for the chance of something going exceptionally well (perhaps on 01-09 the tumbler takes no damage, and 10-30 half damage). You just have to decide on what sorts of good and bad outcomes you want to allow for the attempt.

A "to-hit" type of roll would be more relevant if some type of targeting was involved...for instance, trying to throw a grapnel. If you're simply trying to get it to hook on a wall, you could pick a fairly easy AC (say, 7-10) and have the character make an attack roll (modified, if you like, by their Dex bonus for missile weapons). If they want to throw it through a particular window, something tougher to hit (say, AC 4) might be in order. If they're aiming for a small, high up window, you could declare the target as AC 0...or even lower, if it's a really hard shot. (In this case, multiple attempts would certainly be allowed...though the amount of noise might attract guards, so getting the task done quickly would be preferable!)

The idea, still, is to keep the game interesting and keep it moving. Give them a chance of success, come up with a reasonable penalty for failure (and try to be at least somewhat forgiving about it, unless a character is attempting something truly harebrained or risky), and let the dice fall where they may. Allow the players to describe mitigating factors or ways in which they will attempt to bolster their chance of success, and nudge the chances a little more in their favor if their ideas sound good. Be flexible, be creative, and have a sense of humor about it...and always remember that while the GM is in principle supposed to be neutral, it never hurts to root just a little for the PCs...after all, your adventure won't get very far without them.

A useful thing to consider is what kind of game you want to run. What are the expectations for the action in the game? Is it brutal, slog through the mud, slip on your knees and beat someone to death with a shield? Or is it swashbuckling with running along bannisters and swinging on chandeliers?

Figure out what's a reasonable expectation of what you want to happen in play, let players know what that is, so they know what kind of stunts make sense to use.

Within that, it's also worth considering what scale the "heroes" are compared to the rest of the world. I recall old D&D has 1st level Fighters listed as "Veterans". What a veteran soldier is good at, the kinds of knowledge they have, puts them above the average warrior. So think for a bit if that's going to be the case in your game, or if they're actually just very talented rookies.

Skill Checks

When you know what's reasonable, you can start figuring out what you want to use for a skill check. The most common method is rolling equal or under an attribute stat, usually with a D20 and modifiers. Another suggestion is to use variable dice as the difficulty - "Oh, you're doing something easy? Roll 2D8 equal or under. You're doing something really hard? How about rolling 3D10 equal or under."

Another common method some used is Saving Throws, though it can be a bit of a reach given that older Saving Throws are weirdly specific - "Uh, I guess out running this guy is like being fast enough to dodge a wand..."

Another is simply GM fiat - the player says what they're doing and you decide if it works or not. This can work real well if the players and you are both on the same page of what is an acceptable, or reasonable sort of action.

One of the OSR games, Castles and Crusades uses a simple system of two target numbers - you use the lower one if your class/race would be particularly skilled in that activity, and you add your level.

The Double Edged Sword of Non-Design

Some folks praise the lack of a system as "true freedom", but having been that kid who had to learn D&D from a box, not having real rules or examples of how to do stuff like the Old School Guide back then, mostly meant seeing lots of PCs die from the punishing mechanics. The strength is that you can basically use whatever system or set of odds you like - the drawback is you end up having to design it, and if you don't know what you want out of RPGs, it's pretty much flying blind.

I generally prefer options that demand player input and choice - their odds of success or results of action are better with clever choices, rather than relying primarily or solely on their stats. Figure out what level of action-y heroism fits for play, let the players know, and give them some leeway to take back actions until they're comfortable with it:

"I jump on the table and kick him in the face!"
"Actually, that's pretty hard, you're in armor..."
"Oh, geez. Uh, can I just topple the table over and push it on to him instead?"
"Yeah, that's an average strength check."
"Cool."

Historically a popular method of dealing "skill checks" has been an attribute roll. Methods include

The use of an attribute as a percentage. For example an intelligence of 15 means a 15% chance of success

Multiplying the attribute by a factor (typically 5). For example example an intelligence of 15 means a 75% chance of success

Rolling a d20 equal to or under the attribute.

Rolling a d6 or d20 above some number with the attribute providing a bonus. Typically for AD&D first edition common modifiers are 15 = +1, 16=+2, 17 +3, 18=+4.

Use the secondary skill system in the DMG on page 11. Unfortunately it gives little guidance on how to resolve use of a secondary skill.

Another broad method is to obtain the Wilderness Survival Guide and the Dungeoneer Survival Guide and use the skill system in there. In a nutshell, the weapons proficiency system has been extended to cover non-weapon proficiency. Checks are resolved on a d20.

These example are taken from the Dungeon Tac Cards published by Judges Guild

While published both rules were originally taken from rulings made in Bob Bledsaw's, the owner of Judges Guild, campaign.

What I used, fleshed out in the Majestic Wilderlands book I wrote, is based on a d20 roll. For most tasks the base chances of success was 15 or better. Roll was modified based on an attribute modifier. If the task is judged to be very difficult the chance of success is 20+, if is trivial or easy only a natural 1 failed.